Refractory investment and mold



Patented June 15, 1954 Thomas E. Moore Ohio, assignors 6 Claims.

This invention relates to refractory investments particularly useful in the casting of metals melting over-2390 F. particularly dental castings of the chroma-cobalt type, to investment compositions useful in producing such investments, to molds made with such investment compositions, to methods of making such compositions, investments, molds, and castings, and their utilization.

The invention is particularly concerned with such compositions, articles, and methods desirably used in the manufacture of smooth precision castings of intricate shape by the so called lostwax method. This type of method may well be illustrated by the steps used in making dental castings generally as follows:

1. An impression is taken of the mouth with an elastic type impression material.

2. A master model is made by pouring a mix of dental stone (calcium sulfate hemi-hydrate composition) into the impression. 3. An impression is taken of the master model, using a duplicating material which is usually a gelatin base. (Korogel or hydrocolloid materials are sometimes used, but gelatin base impressions have been most satisfactory.)

4. A mix of the investment material is made and vibrated into the impression.

5. After the investment has set, it is removed from the impression and wax is flowed and adapted onto the model and around the teeth to form the clasps and the remainder of the dental appliance that it is desired to reproduce as a metal casting.

6. The waxed up model with adequate sprues attached is then placed in a flask which is filled with an outer investment.

7. After the outer investment has set, the flask is fired to 1400" F. or higher to remove all traces of Wax and moisture and to expand the mold.

8. Molten metal is cast into the hot investment mold, usually with a centrifugal casting machine.

9. The casting is removed from the investment material.

In carrying out such methods, various types of investment compositions have been used. Thus an investment material consisting essentially of silica flour and bonded by an acidified organic silicate such as ethyl silicate is well known in the art. Further it is known to use investment materials that are bonded by mixing with dilute acids. These compositions have presented certain difiiculties however which have been more or less objectionable in their utilization. In the case of the ethyl silicate bonded investment, it

REFRACTORY INVESTMENT AND MOLD and Claude H. Watts, to The Ransom and Randolph Company, Toledo, Ohio, a corporation No Drawing. Application March 2, 1950, Serial No. 147,328

Toledo,

of Ohio is necessary to tamp the investment for several hours after pouring up the model in order to compact the mold so that it will be hard and dense enough for practical use. The bond is also very slow to set up for the model to be hard enough to work on. The tamping operation is objectionable because it is awkward, clumsy, and time consuming-and in many instances is likely to be deleterious to the accuracy of the finished casting. Investments mixed with dilute acids have been unsatisfactory because they do not at tain suflicient strength on setting at room temperature to make a good model. Also the surface of the investment next to the model does not separate cleanly, but leaves a rough surface which is not an accurate reproduction of the impression taken in the mouth.

Other difiiculties have been experienced with other types of investment materials. Thus calcium sulfate bonded investment decomposes'at the temperature of the high melting metals to which particular reference has been made above.

Among the objects of the present invention is included refractory investment material for casting partial and full dentures of high fusing alloys.

Other objects include the utilization of two different types of investment materials in the production of a mold one composition particularly being used for the model investment and the other for the outer investment as more particularly further described below, the two investments varying in properties to give complete control of the operation and elimination of difiiculties of the priorart.

Other objects of the invention include a model investment that will set rapidly and attain sufficient surface hardness and strength without tamping so that it may be removed from the duplicating material in approximately one hour or other relatively short space of time without breaking the teeth or other parts of the model, producing a clean sharp cast.

Still other objects include investment of sulficient hardness to permit handling and manipulation during the waxing operation without scratching or other impairment of the surface by waxing instruments used or by the fingers of the manipulator.

Still further objects include investment materials of sufiicient setting and thermal expansion to compensate for the casting shrinkage of the metal.

Still further objects and advantagesof the present invention will appear from the more detailed description set forth below, it being understood that such more detailed description is given by way of illustration and explanation only, and not by way of limitation, since various changes therein may be made by those skilled in the art withoutdeparting from. the scopaandspir-it of the present invention.

In the present invention one of the particularly emphasized features is the discovery that: it has been found desirable to use two separate and distinct investment materials one-- for" the model investment and the other. asanouter in.- vestment, each investment. having controlled physical properties, and the investment: come positions being particularly compounded for use with each other, in the production of refractory molds which solve many of the difficulties in-theart. The utilization of two different types of investments namely a model investment and an outer. investment. enables the. properties oi each of'the. investments tobe. controlled togive. the most-desirable properties for the particularpurpose or. function for whicheachinvestment. is adapted andyet enabling theinvestment'stobe formulatedfor. use with each. other in order. to secure. the objects. sought. While particular types, of investment. compositions, are. herein? after. set. forth as most particularly, valuable. for present purposes, there are. certaingeneraLcQnsiderations. and relations between. the. two types of, investments that are particularly important in.the.casting ofjthehighfusion metals. as. set forth above for thesepurposes. Thusit hasbeen found that the-outer investment neednotbeas hard as themodel investment andtherefore. the outer investment may exhibitalower. compression strength than the .model'investment. It has also been found that the setting expansion. of

the outer investment should not be morethan. .2.%

andthat the thermal expansion ofthe outer investment should not be more. thanthe thermal expansion of. the model investment. These expansion limitations are necessary to preventthe. outer. investment from expanding away, from the model, which would obviously result 111.1111? desirable. flash. on.the finished casting. By. the productionof. two different. typesof investment materials insofar. as such setting and expansion propertiesare concerned, they may. be. controlled to give the most desirable set of properties. in.the particular investment considering. the purpose which it serves andthe manner in which it is used; 7

The investment" compositions employed. in the production of the model investmentand'of' the outer investment may be substantially-different in character as long as they possess the essen tiali characteristics in their relation one to the other as pointed out above and exhibit otherwise; properties enabling theirw utilization in-"the casting of metals which meltabove .2300? But desirably; both the: model and outer. investment compositions may generally be made ofthesame types of materials'within the limits and with modifications as set. forth below to. adjust the properties thereof for the purposes set. forth above.v The most desirable types of, materialsthat may be, utilized as the. basis fortheinvesta ment compositions for both the-model. andouter. investments may be produced from compositions whichcontain a major amount. of. a refractory material ianda minor, amoun t. of. binding. andsetting agents such as Water soluble acid phosphate. salts andbasic material. utilized therewith. The

ing available in the art.

refractory material employed particularly. for dental purposes will generally be silica particularly compositions made up of graded silicag f different mesh using for example a balan ed composition of silica running from about: 30 mesh, through 50 mesh, 106 mesh and 200 mesh for. example to. give the. desired. graded composition for such: purpose. Qther refractory mat rials may be employed, many such materials lee- The silica may begin any of its various forms such as quartz, trieiymite, cristobalite; and also hydrated forms suich as, silicic acid,. tripoli, diatomaceous earth, cite. Silica. is the, most, desirable refractory material because it is;v inexpensive and because it forms satisfactory high temperature bond with the; phosphate. Other oxides of the silicon group maybe used in place of silicon dioxide either in wholeor; in part such as the oxides of titanium, zirconium, and thorium; and other refractory materialssuchas nonplastic refractory clays, .fire clay, kaolinite, silli'manite, andalusit'e,,mullite, silicon carbide, zirconium. silicate, titanium. sillicate, the spinels, refractory aluminumoxideetd, mayjbeused either in whole or. to replace at least a part ofthe silica if desired,;or various combinations of these dff'erentLrefractcry materials may be utilized, particularly for the outer investment. The model investment. should exhibit. a high thermal expansion, so that the. refractory used therein should be selectedwiththis in view; andaccordingly may. desirably consist, chiefly of silica or cristabolite. Additions. of; other silie catesror refractory metal oxides. should'be held to a minimum, or used in;the smallest per cent possible to give the desired property. For ex ample the addition of" zirconium silicate to, the model investment. actually reduces the thermal expansion, but thesurf ace .hardnessandstrength are considerably increased, The. invention however will be particularly illustrated by the use of silica.

A water: soluble acid phosphate salt is desirably used in the composition particularly bal anced mixtures of such. acidphosphates. Compounds producing P205 at elevatedutemperatures maycomprise acid" salts of" ortho, meta, and pyrophosphori'c. acid with lithium, magnesium, calcium, ammonium, strontium, barium, copper, zinc; cadmium, etc. Also, besides ammonium phosphate, there may be used the organic amino compounds such as primary, secondary and' tertiary'methyl, ethyl and rr-propyl amine phosphates. However, any large quantity oi ammonia orother gasdriven off during heating is Of these various alkali metal and undesirable: alkaline earth metal salts; etc;, mono ammonium phosphate andmono magnesium phosphate are preferred because they ar available incom mercial: quantities. r

The easier-material should be carefully chosen forreactionwith the phosphate: togive. the come positionrthe desired setting and other properties. Basic materials including the oxides and. hydroxides of calcium, strontium; zinc, cadmium, copper, lithium, and. barium, give investments whichare, too soft for. theepurposes of thisinvene tion.. Fused. magnesium oxide. is, therefore. def sirably: used.

Considerations governing the these-components of the investmentcompositions and. other features involvedinthe inventioncan hestbeieft until after. a consideration of formulations that may be desirablyempioyed andillusxtrateimportant features of .the present. invention.

utilization ofv 7 qfillustrate the composition used for the model inv stment the following is given:

FORMUISLA FOR MODEL INVESTMENT fused magnesium oxide325 mesh %i powdered mono magnesium phosphate powder-ed mono ammonium phosphate 20% milled zirconium silicate-325 mesh 20 Z 6 silica-50 mesh 1 silica-3D mesh silica-2'00 mesh 'iIhis investment composition has a refractory atin size gradation such that a minimum of iquid is required to make a putty-like mix that Jill flow under gentle vibration.

Representative formulas for the model investment' illustrating limits for the fused magnesium zOXidWhiCh desirably may be from to and limits for the soluble acid phosphate salts which desirably may be from 1.0% to 17.5% are as follows:

No. 1 (Low limit) 5 fused MgO-325 mesh 4% Mg(H2PO4) 2 20 milled zirconium silicate-325 mesh 20 50 mesh silica 10 30 mesh silica 35 200 mesh silica W/I (water to investment composition) 10/100 liquid 13% silica 501 NO. 2 (High limit) 10.0 fused Mg0325 mesh 4.5% Mg'(H2PO4) 2 13.0% NH4H2PO4 20.0% milled zirconium silicate325 mesh 20.0% 50 mesh silica 10.0% 30 mesh silica 22.5% 200 mesh silica W/I=10/100 liquid 18% silica sol Formula. No. 1 may be used with some success, but the strength is somewhat lower than that required for a good model, and the teeth are apt to break off on removing the model from the duplicating material. Also the model tends to develop small cracks on setting at room temperature after removing from the duplicating material. Formula No. 2 is very satisfactory and although higher percentages of fused magnesium oxide and phosphate salts could be used with some success, the thermal expansion of such formulas would not be sufficiently high and the resulting castings would be under size.

It is not absolutely necessary to use zirconium silicate (zirconite, etc.) in the model investment. A satisfactory and workable formula can be compounded using silica only, in varying mesh sizes, for the refractory filler as taught herein. How-' ever, the preferred formula does contain 20% zirconium silicate because the surface hardness is increased, and from 5% to 25% of zirconium silicate may be used, desirably for this purpose. Other refractory materials among those mentioned above may be used to replace zirconium silicate in whole or in part with similar beneficial results.

It has been found that in lieu of using water for addition to such composition to produce the investment material, superior results are obtained by the utilization of silica sol such as that manufactured by National Aluminate Corporation or by Du Pont under the name of Ludox.-- Such silica sols are represented by those setforth in U. S. Patent'No. 2,244,325. Silica sol usedin this way has been found to have a very important efiicacious effect in the nature of the investment produced particularly from compositions of the character set forth above and modifications thereof as hereinafter taught. The use of silica sol in this way particularly within restricted concentrations will increase the setting expansion and also increase the hardness of the model, which makes for a smoother, harder surface both of which are desirable properties. Silica sol is available commercially either in 18% or 30% concentration, and by dilution any concentration of silica sol up to 30% may be used with corresponding variations in expansion properties, hardness, and compression strength. In some applications a silica sol concentration of as low as 2% concentration has shown a noticeable increase in setting expansion and hardness. While the 30% silica sol concentration may be used in some applications, generally the hardness and density of the investment when using such 30% silica sol concentration is increased to the point where the investment is not sufficiently permeable to allow the metal to cast out during the casting operation and gas pockets may be found in heavy sections, Consequently in general the concentration used particularly in connection with the casting of metals in accordance with the present invention is less than 30% for example between 2 and 21% and particularly important effects are obtained with concentrations of between 10 to 21% and specifically by the use of a solution of silica sol of 18% concentration. Specifications for 30% and 18% silica sol are as follows:

To illustrate the ratios in which it is used, the

., formulation as given above (No. 1) for the model investment when mixed with 18% silica sol in the proportions of 10 c. c. of 18% silica sol to grams of model investment, gives the following physical properties in the investment:

Initial setting time (Vicat needle) 6 mins. Compression strength, 2 hrs 2000 lbs. per sq. in. Setting expansionMax.

3 hrs. .85% Thermal expansion-Max.

The following table shows setting expansion of Investment, Formula No. I for varying per cents of S102 sol in mixing water, using liquid-investment ratio of 10 c. c. to 100 grams:

21% s10, Sol ..:IIIIIIIIIIIIII..IIII

This chart shows an increase in setting expansion as the per cent SiOz sol in the mixing liquid is increased up to a maximum for 18% $102. A further increase'in per cent S102 to 21% gavea lower-setting expansion than the 18% concenassesses compositions. will for: the same lilifierenir. investment. varying: setting: expansions: concentration of: silicav sol..

For: example; the. following: formula, when xnixedzthe; proportion. of 110 c;. 0.. liquid. to. 100 grams-investment, gave. setting expansions. shown below.

Bercent SiOz. 'PercentS-et; Sol. Exp

Formula 3 smoother surface onthe cast metal and better fit.

"llhese: considerations will show the excellent resultsobtainedby the use character. herein set forth.

To illustrate considerations in connectionwitn the outer investment, the following composition is illustrative:

II. FORMULAFOR OUTER'INVESTMEiNT' 8% fused magnesium oxide-32'5mesh 9% powdered monoammoniumphosphate 5.5 powderedmono magnesium phosphate 27% 501- mesh silica V 25% 100. mesh. silica 2515 200 meshsilica This" formulation when mixed with water in the: proportions for example of 18 c. c. water to 100.:gran1s of outer investment gives the following physical properties in-, the resulting investment:

Initial setting time (Vicat needle) .10 mins.. Compression strength, 2 hrs 1140 lbs. per-sq. in; Setting" expansion; maximunn .04 7; Thermall expansions; .30'%

Representative formulas for the; outer: invest? mentiillustratingalimits for: the fusedmagnesiumzoxide which: desirably may be-irom: &% to 10%:

ofthe silicasolwith investment compositions particularly those of the: However such; silica; sol may'be desirably used as the. bonding-.mataand. for th-SOIllb1BP Eflidi phosphatex fl which desirablymay be: from. 1.25735 tm lii are. as follows;

N o. 4. (Low limit).

27.00% 50 mesh silica 25.00% 100 mesh silica 36.75% 200 mesh silica W-'/I=18'/100 using water for mixing No; 5 (High: limit) 10.0%. fused..Mg.O-325' mesh 27.0%. 50. mesh silica 25.0 L00 meshi silica 19.8% 200 mesh silica W/I=l8 100 using" water for mixing As explained above it is important to produce an outer investment showing a controlled setting, expansion 01 not over. 2%.. It. has beenfbundpossible to make an investment that. will meet this setting expansion requirementby. contr-olot the balance between the alkaliused and. 131387591111 ble acid phosphate salts in compositions of the character particularlyillusifrated above andmore specifically'whenfused magnesium oxide is employed as the alkaline"; material: for which it is most desirably suited as against the other alkaline.

" materials although the" latter maybe emplbyedi An important consideration in producing such low setting expansion with compositions of this type is in theutilizati'orr of sufiicientr fused mag nesium oxide for example so that there wilt be an excess of such oxide afterallof the soluble acid phosphate salts have been neutralized. In the particular formulation as given above'calcut lation will show that for 100 grams of material of'the iormulationthere given for. the outer in!" vestment, 6176 grams of M'gO are required for neutralization of. the soluble acid phosphate. salts present whereas 8 grams-are present in the for..- mulation so.that there is an.excess of' 1.2!; grams of magnesium oxide per 100' grams. of the. stated additive investmentcomposition. above that neeessary for neutralizationoi all oi the soluble acid phosphate salts. present. The. excess magnesium. oxidewhich may bepresentfor. this. purpose be, yondltliat necessary; torneutralize all. of the some ble acid phosphate saltsin such. compositionsmay be..varied but.should. not .be excessive; For examplesatisfactory. results. are obtained if theamount. of; magnesium: oxide. in excess of. thatnecessary to. neutralize. the. phosphate. salts present. is fromno 3%.. It.is.not.necessary to usev zirconium. silicate in theouter investment, .but it can'be used if desired, and: offers one way to control-the thermal expel-nesionso-thatit-wouldnot-be greater: than that-of the. modelinvestment.

strength is not required, .andthe. higher cost-more? However, the 7 added! or, less discourages' its use here. 7

With regard to particle size of the fused.mag nesia for use'bothin. themodelinvestment and in the outer investment-,both' sizes may, desirably beminus 325 mesh, i. e..10.0% through-325-meshz Diiferences between the particle size" inthe: dif'-= ferent investments. require a determination" of: particle size; below. .5. micron, in; connection: with. which itmay 'benoted that there-issue generally.-

the fused magout above as de- Another dif- .fe'erence preferably used is in the amount of water (employed in making the mix. A ratio of c. c.

liquid to 100 grams of model investment gives a very thick mix, which in turn makes for a harder and a'stronger model. The consistency is such that it will flow under slight vibration. A ratio of '18 c. c. water to 100 grams of outer investment gives a fairly thin mix the waxed up model. The consistency is such that the investment can be poured without vibration. It should be noted that the stated ratios of water or silica sol, as the case may be, to investment composition, as given above, are optimum ratios. These ratios may vary in each investment material but those set forth give the best effects.

The ratios and proportions of the components of the investment materials as exemplified above, and generally in the ranges of from 1-10% of magnesia, from about 7-7.25 to 18-18.2%, balance refractory (all ratios by weight), should in all cases be balanced to give the desired properties in the finished product. Ordinarily the fused magnesium oxide and soluble acid phosphate salts should be in the ratio of about 4:7 specifically 4:725, for the best results, but variation in this ratio within limits as herein taught may be used where the optimum effects are not essential. In addition, much better results are obtained by using a balanced formula of mixture of acid phosphate salts than by the use of a single acid salt in the composition. For example mono magnesium phosphate alone or mono ammonium phosphate alone is not as satisfactory as a mixture of them in balanced proportions. Generally speaking, in such a mixture, the mono magnesium phosphate should comprise from to 40% by weight of the soluble acid phosphate salts present.

As overall values, the outer investment composition should contain by weight a major amount, not less than 70% of refractory material and a, minor amount, not more than 20% of water-soluble acid phosphate salt, the balance being the magnesium oxide. More specifically there will be from 89% to 71.8% of refractory material, from about 7% to 18.2 of water soluble acid phosphate salt and from 4 to 10% of magnesium oxide.

For the model investment composition the preferred ratios are from about 72.5% to 85% of refractory material, from about 17.5% to 10% of water soluble acid phosphate salt, and from about 10% to 5% of fused magnesium oxide. And as stated with the model investment composition a silica sol of from 2 to concentration by weight is desirably used as the liquid for addition to the composition.

The utilization of silica sol as set forth herein in phosphate type investments gives important results as pointed out above. Silica sol may also be used with other phosphate type investments particularly where the refractory material is that can be painted onto bonded by the use of acid phosphates in combination with basic material. Thus silica sol may be used with any of the investment compositions as set forth in Patent No. 2,479,504. It may be used in those combinations of the patent by employing as liquid for admixture with the solids, a silica sol of from 2 to 30% concentration by weight preferably using from 10 to 21% as also set forth above. In this way there is obtained increased hardness and increased setting expansion, giving smoother surface on cast metal and better fit.

Having thus set forth 1. In combination,

our invention, we claim: a refractory mold for use in casting metals melting at over 2300 F. comprising a model investment, a wax pattern one face of which is in contact with said model investment, and an outer investment in contact with another face of pattern, the outer investment differing in composition from the model investment and having a lower compression strength than the model investment, a thermal expansion not greater than that of the model investment, and a setting expansion of not more than 2% the model investment consisting essentially by weight of from 89% to 71.9% of a refractory material, from 7% to 18.2% of a water-sob uble acid phosphate salt and from 4% to 10% of magnesium oxide, in which 5% to 25% by weight of the refractory material is zirconium silicate, and the outer investment consisting essentially by weight of from 89% to 71.8% of a refractory material, from 7% to 18.2% of a water-soluble acid phosphate salt and from 4% to 10% of magnesium oxide, the outer investment being substantially free from zirconium silicate.

2. A combination as set forth in claim 1, in which the model investment is bonded with silica sol of from 2 to 30% concentration in the ratio of 100 gms. of model investment composition to 10 c. c. to 18 c. c. silica s01.

3. In combination, a refractory mold for use in casting metals melting at over 2300 F. com- 3 prising a model investment and an outer investment, the investment compositions being difierent but each consisting essentially of by weight from 89 to 71.8% of a refractory material, from 7% to 18.2% of a water soluble acid phosphate salt, and from 4% to 10% of magnesium oxide, the ratio of salt to oxide being about 7:4 in which 5% to 25% by weight of the refractory material of the model investment is zirconium silicate, the outer investment being substantially free from zirconium silicate.

4. A model investment for use in casting metals melting at over 2300 F. comprising by weight from 72.5% to of a refractory material, from 17.5% to 10% of a water soluble acid phosphate salt, and from 10% to 5% of fused magnesium oxide, and in which from 5% to 25% by weight of the refractory material is zirconium silicate, bonded with silica sol in the ratio of gms. investment composition to from 10 c. c. to 18 c. c. silica sol.

5. An investment composition comprising a setting refractory magnesium oxide-water soluble acid phosphate investment material bonded with a silica sol in the ratio of 100 gms. investment composition to from 10 c. c. to 18 c. c. silica sol of 230% concentration to give a workable slurry.

6. In the process of making a casting by the lost-wax method from metals melting at over 2300 F., the steps which include producing a cast 1 1 of the desired articlein a'modelinvstinent eonsisting essentially of by Weight from 72.5% to 85% of a refractory material, from 17.5% to 10% of a water soluble acid phosphate salt, and from 10% to 5% of fused magnesium oxide, in which 5% to 25% by weight of the refractory material is zirconium silicate, coating said cast with wax, and applying an outer investment to said waxed up model investment, said outer investment consisting essentially of by weight 89% to 71.6% of'a refractory material, from 7 to 18.2% of a Waterso1ub1e acid phosphate salt, and from 4 to 10% of magnesium o'xidejsaid outer investment being substantially free from zirconium silicate and differing in composition from the model investment and having -a lower compression strength than the modelinvestment, a thermal expansion notgreater than that of -the model investment,

and a setting expansion of not more than 2%.

12 Ref'fiis ci'tea the "file or pate UNITED "STATES PATENTS Number Name Date R Dailey Mar. '14, 33 Birdlebough- Qct. 13,1 36 Moosdorf Mar. 2,1 3-7 Erdle I Apr. 2, 1940 Neiman July '1, 15941 Lee Nov. "2, 1943 M111er'. Feb.'6,1945 Wainer Mar. 27, 1%45 Collins 'Aug. 7,1945 

1. IN COMBINATION, A REFRACTORY MOLD FOR USE IN CASTING METALS MELTING AT OVER 2300* F. COMPRISING A MODEL INVESTMENT, A WAX PATTERN ONE FACE OF WHICH IS IN CONTACT WITH SAID MODEL INVESTMENT, AND AN OUTER INVESTMENT IN CONTACT WITH ANOTHER FACE OF SAID PATTERN, THE OUTER INVESTMENT DIFFERING IN COMPOSITION FROM THE MODEL INVESTMENT AND HAVING A LOWER COMPRESSION STRENGTH THAN THE MODEL INVESTMENT, A THERMAL EXPANSION NOT GREATER THAN THAT OF THE MODEL INVESTMENT, AND A SETTING EXPANSION OF NOT MORE THAN .2% THE MODEL INVESTMENT CONSISTING ESSENTIALLY BY WEIGHT OF FROM 89% TO 71.9% OF A REFRACTORY MATERIAL, FROM 7% TO 18.2% OF A WATER-SOLUBLE ACID PHOSPHATE SALT AND FROM 4% TO 10% OF MAGNESIUM OXIDE, IN WHICH 5% TO 25% BY WEIGHT OF THE REFRACTORY MATERIAL IS ZIRCONIUM SILICATE, AND THE OUTER INVESTMENT CONSISTING ESSENTIALLY BY WEIGHT OF FROM 89% TO 71.8% OF A REFRACTORY MATERIAL, FROM 7% TO 18.2% OF A WATER-SOLUBLE ACID PHOSPHATE SALT AND FROM 4% TO 10% OF MAGNESIUM OXIDE, THE OUTER INVESTMENT BEING SUBSTANTIALLY FREE FROM ZIRCONIUM SILICATE. 